Field of the Invention
This invention relates to the field of photo- and videography.
Description of the Background Art
Conventional mobile devices have a wide range of functions. One of the most popular functions is the recording of photographic and video material. In order to take photographs and record videos with mobile devices, units called camera modules are used. A camera module incorporates a photosensitive sensor, optical-system lenses, and, in some cases, components for a focusing system.
In recent years the technical characteristics of camera modules in contemporary mobile devices have improved appreciably. Theoretical limits have virtually been reached for optical quality (optical resolution is constrained by the diffraction limit) and for sensor performance (the conversion of photons into an electric charge is reaching a level close to 100%). There is a further limit, namely, the minimum dimension of an individual photosensitive element in the sensor (pixel). In contemporary sensors the size of the pixel has reduced to 1 μm, which is comparable to the wavelength of the light being detected. A further reduction in the size of the pixel will not lead to an increase in the image resolution because of the proximity to the diffraction limit. Limits on the use of space have also been reached. The length and width of the camera module are limited by the dimensions of the photosensitive sensor, and its height is limited by the lens system (camera lens). To produce a high-quality and clear image, complicated camera lenses are needed that comprise a large number of separate elements.
Further, improvement in the quality of the recorded image is not possible without increasing the transmission of light to the photosensitive element (sensor). The maximum amount of light reaching the sensor is limited by the diameter of the inlet aperture of the optical system. An increase in the inlet aperture requires an overall increase in the geometric dimensions of the camera module. The geometric dimensions are, in turn, limited by the mobility requirement. The greatest limitation relates to the height (the height of the camera module must not appreciably exceed the overall thickness of the base device). But the height of the camera module depends on the overall thickness of the lenses in the lens system and the necessary gaps between the lenses.
The following is a list of fundamental problems that result from decreasing the amount of light entering camera modules on mobile devices:                High levels of noise in the resulting image decrease the overall visual quality.        In low lighting, it becomes necessary to use long exposure time, which causes blur. Since cameras are typically handheld, photographs blur not just as a result of object motion but also because the camera itself is moving.        Moreover, small camera modules necessitate proportionately small pixels on the sensor. As a result, the pixel can hold only so much charge before draining, which means that the camera cannot flawlessly capture an image with a high dynamic range that most scenes present.        
The following technical solutions are available to improve the quality of images produced with camera modules on mobile devices:                1. Array camera. By using several camera modules grouped together on one device, it is possible to increase the amount of light intake by combining the charges from all sensors, for example, from US Pub No. 2011/0080487 which corresponds to U.S. Pat. No. 8,866,920 B2, Capturing and processing of images using monolithic camera array with heterogeneous imagers, Kartik Venkataraman, Amandeep S. Jabbi, Robert H. Mullis, Jacques Duparre, Shane Ching-Feng Hu).One drawback of this technique is that the overall cost of camera modules will increase proportionately to the number of modules. The amount of space taken up by camera modules will also increase.        2. Effective noise-reduction algorithms. Significant progress has been achieved recently in separating image from noise in noisy images described in U.S. Pat. No. 7,835,586, Method for filtering images with bilateral filters, Fatih M. Porikli.This method is severely restricted by the limit of the signal-to-noise ratio. Low-contrast details are impossible to separate from noise. That is, they are lost during noise reduction.        3. Multiframe exposure. By taking several photographs in rapid succession and combining them, image noise can be reduced, the dynamic range can be extended, and resolution can potentially be increased as disclosed in U.S. Pat. No. 8,699,814, Method for improving images, Dmitry Valerievich Shmunk, Eugene Alexandrovich Panich; US 2013/0156345, Method for producing super-resolution images and nonlinear digital filter for implementing same, Dmitry Valerievich Shmunk.Combining several frames, however, increases total exposure time, causing problems that cannot be entirely avoided, such as moving objects appearing blurred.        
The method for reducing the overall size of a camera by using a folding design has been known, for example, from U.S. Pat. No. 1,435,646, Folding camera, Robert Kroedel.
One disadvantage of this design is that its simplicity entails the use of small-aperture optics (apertures used are f/8-f/11). Any attempt to use a folding structure with the system of a modern high-aperture (f/4-f/2) and high-resolution camera lens leads to an extremely complex design whose drawbacks manifest in its brittleness and the need to allocate additional space for this system. Modern camera lenses consisting of a plurality of component lenses are exceptionally demanding in terms of the precise placement of the lenses and the photosensitive element relative to one another (alignment, distance, etc.). Adding a complicated folding structure inevitably leads to a substantial decrease in the precision of their placement. In addition, the use of a folding structure in a mobile device is impossible because in most cases, the optical system is surrounded by a focusing system, which is typically a monolithic structure and does not allow the optical system to be folded.
A camera module is known, for example, from Eric J. Tremblay, Ronald A. Stack, Rick L. Morrison, Joseph E. Ford, “Ultrathin Cameras Using Annular Folded Optics,” Applied Optics, vol. 46, no. 4, 1 Feb. 2007, in which a pair of specially shaped mirrors replace the set of lenses in the traditional optical system. The mirrors are placed opposite each other. Because of the multiple reflections between the mirrors in the system, the optical path is substantially greater than the distance between the mirrors. This method can significantly reduce the thickness of the camera module while increasing the amount of light reaching the photosensitive element. An additional advantage of this solution is that it is free from chromatic aberration because it uses only reflective elements.
A disadvantage of this solution is its low depth of field. This is because light comes into the optical system through a narrow rim around the perimeter of the mirrors. Another disadvantage is the low angular field of view captured by this solution. It is not possible to create the most demanded type, a wide-angle camera module.
The closest technical solution to the proposed (prototype) is described in Felix Heide, Mushfiqur Rouf, Matthias B. Hullin, Bjorn Labitzke, Wolfgang Heidrich, Andreas Kolb, “High-Quality Computational Imaging Through Simple Lenses,” ACM Transactions on Graphics, vol. 32, no. 5, 2013, 10 pages, ISSN:0730-0301, EISSN: 1557-7368, DOI: 10.1145/2516971.2516974, which uses a simplified optical system consisting of a single lens allowing the weight of the camera module to be reduced considerably. Aberrations that inevitably arise as a consequence of using a single optical element are offset by subsequent digital processing of the resulting image.
A drawback of this solution is that it cannot be used on mobile systems since replacing several optical elements with a single element does not reduce the size of the camera lens. Consequently, the overall size of the camera module does not change. It is possible to enhance the compactness of this optical system by adding known folding designs used in photography; however, this does not produce the required result since to achieve the required lens speed in this solution, the lens would need to be sufficiently thick.
Accordingly, the aim of the proposed solution is to create a mobile device that would use a camera module allowing high-quality images to be produced, particularly in low-light conditions, while retaining the small dimensions of the device.